To improve the rigidity (or stiffness) of a tire, usually a high hardness reinforcing rubber (bead filler) extending from the bead along the tire sidewalls is provided at the inside of the sidewalls. In this conventional high hardness rubber composition, since the unvulcanized viscosity is high, sometimes the temperature rise at the time of extrusion is large, flaws, scorching, and other problems are caused, and the tire shaping precision in the later steps is degraded.
To further improve the good processability, high scorch resistance, etc. of high hardness rubber, the technology has been proposed in that particles of polybutadiene rubber having a glass transition temperature of −60° C. (specifically, the rubber gel having a toluene swelling index of 1 to 50) into a rubber having a double bond (specifically, natural rubber (NR), styrene-butadiene copolymer rubber (SBR), polybutadiene rubber (BR), etc.) (Japanese Patent Publication No. 2001-354807A). This Publication discloses that the die swell decreases due to this compounding. However, polybutadiene rubber gel particles are insufficient for the object of the present invention.
Further, as the basis for the present invention, Japanese Patent Publication No. 2002-60437A discloses a novel conjugated diene-based rubber gel having a toluene swelling index of 16 to 70 and a rubber composition obtained by compounding this rubber gel into a rubber cross-linkable with sulfur and exhibiting an excellent abrasion resistance and low heat buildup property, while maintaining good mechanical properties. However, this prior basic invention does not propose technology for compounding the rubber gel into a diene-based rubber along with a specific amount of carbon black to improve the fluidity and dimensional stability in the high hardness reinforcing layer of the bead filler at the time of extrusion and further does not propose technology for compounding this rubber gel into a diene-based rubber together with a specific amount of polybutadiene rubber containing a cis 1,4-structure in 90% by weight or more to improve the extrudability and dimensional stability at the two ends of the tread extrudate.
Further, International Publication WO 02/000779 discloses a rubber composition having a good processability containing cross-linked rubber particles, having a small rolling resistance and having an excellent wet skid resistance and good abrasion resistance and tensile strength etc. However, the cross-linked rubber particles of the prior invention are obtained by a high polymerization conversion rate, and therefore, there are the problems that there are many toluene insolubles, sufficient mechanical properties cannot be obtained, the flex fatigue deteriorates, and the improvement in the abrasion resistance is insufficient.
A tire tread is usually manufactured by extrusion, but there are the problems that, at the time of extrusion, the tread ends often crack and the tire shaping precision deteriorates in the later steps. To improve this extrudability, the technique of compounding fine crystals of 1,2-syndiotatic polybutadiene etc. is known, but when used for parts with large deformation such as the tread ends or sides, the durability often became a problem.
Further, Japanese Patent Publication No. 10-204217A discloses the use of a rubber composition obtained by compounding an SBR rubber gel (toluene swelling index: 1 to 15) into a diene-based rubber for a tire tread. However, an SBR rubber compounded into this rubber composition has a low toluene swelling index of 1 to 15, that is, has a high degree of gelation, and therefore, differs from that used in the present invention and differs in purpose of compounding as well.
Further, Japanese Patent Publication No. 2001-139729A discloses using a diene-based rubber composition containing a gelled rubber (toluene swelling index: 16 to 150) for a tire excellent in wet performance and low rolling resistance. However, this invention does not disclose using this gelled rubber together with polybutadiene containing at least 90% by weight of a cis 1,4-structure and arranging the rubber composition at the two ends of the tread.
To achieve both wet performance and low fuel consumption of a tire tread rubber containing silica is widely used. In general, the effect becomes greater the larger the ratio of compounding of silica with respect to carbon black. However, tread rubber, in which a large amount of silica is compounded, has a large die swell, and therefore, the processability at the time of extrusion often becomes a problem. There is prior art for compounding gelled rubber to achieve both a good wet performance and low fuel consumption of the tire. The performance of a rubber composition, in which this gelled rubber is compounded, is governed by the toluene swelling index (Q)—an indicator of gel hardness. As disclosed in the above Japanese Patent Publication No. 10-204217A, when Q is 15 or less, the rubber gel is sufficiently hard and the properties as rubber are almost entirely lost, and therefore, the gel can be compounded as a filler. However, a rubber gel filler having a Q of 15 or less is inferior in mechanical properties compared with the generally used carbon, silica, or other rubber reinforcing agents, and therefore, a further additive is required for practical use (see, for example, Japanese Patent Publication No. 2000-72920A, Japanese Patent Publication No. 2000-72921A, Japanese Patent Publication No. 2000-86825A, Japanese Patent Publication No. 2001-31798A, Japanese Patent Publication No. 2001-89606A, and Japanese Patent Publication No. 2001-187841A). This gives rise to new problems in processability, cost, etc., and therefore, still cannot be said to be sufficient technology. Further, Japanese Patent Publication No. 10-204217A does not disclose using the rubber gel so as to reduce the die swell of a rubber composition containing silica and to increase the compounding ratio of the silica filler.
On the other hand, rubber gel having a toluene swelling index (Q) of 16 or more has performance as rubber and, as seen in Japanese Patent Publication No. 2001-139729A, Japanese Patent Publication No. 2001-354807A, etc., selection of a rubber gel having a suitable glass transition temperature Tg enables tire performance to be improved relatively easily, but rubber gel does not take in any filler, and therefore, if a large amount of filler is compounded, extra mixing time is taken, and therefore, the productivity becomes deteriorating. Therefore, by just compounding a rubber gel, a large improvement in performance cannot be expected. Further, rubber gel having a Q of 16 or more improves the wet performance and fuel consumption, but there is the problem that the abrasion resistance is insufficient for use as a tire tread.
Further, Japanese Patent Publication No. 2001-139729A discloses the use of a diene-based rubber composition into which a gelled rubber (toluene swelling index: 16 to 150) is compounded for a tire excellent in wet performance and rollability. However, this prior invention does not teach the use of this rubber in relation with the ratio of compounding of the silica filler.
Further, the cross-linked rubber particles disclosed in International Publication WO02/000779 contain a large amount of toluene insolubles. In a rubber composition having a high compounding ratio of a silica filler, the problem of processability at the time of extrusion cannot be said to be sufficiently resolved and further the balance between the wet performance and the low fuel consumption cannot be said to be sufficient.
To improve the braking performance, to improve the steering stability and to lower the fuel consumption, the general practice is to use a high modulus, relatively thick gauge undertread (see, for example, Japanese Patent Publication No. 5-178011A, Japanese Patent Publication No. 9-176384A, Japanese Patent Publication No. 9-227720A, and Japanese Patent Publication No. 9-150606A). However, if making the undertread high in modulus and thick in the gauge, a problem arises in processability. For obtaining a high modulus, in general, for example, a large amount of small particle size carbon black is compounded. As a result, the viscosity at an unvulcanization stage is high and the heat buildup at the time of extrusion becomes greater, and therefore, scorching easily occurs during the extrusion. Further, if making the gauge thicker, the heat buildup at the time of extrusion becomes greater, and therefore, again scorching easily occurs. Further, if the viscosity is high, the die swell during the extrusion also becomes high and the extrudate easily shrinks.
Further, Japanese Patent Publication No. 2002-60437A discloses a conjugated diene-based rubber gel having a toluene swelling index of 16 to 70 and compounding this rubber gel into a sulfur-cross-linkable rubber to obtain a rubber composition having good mechanical properties and exhibiting excellent abrasion resistance and a low heat buildup property, but does not disclose the technology for improving the fluidity and dimensional stability at the time of extrusion of high hardness reinforced rubber and the extrudability of the rubber at the two ends of the tread and the undertread.